Can you elaborate on this? I don't understand the figure ground part.
It may not be correct. It seems to be an original theory of mine. I'll explain it and you can see what you think. First the basics.
Figure-ground is a basic concept of perception and you will find enormous amounts of theory and data, most of which I probably don't understand. Very simply, things (figure) either fade into the background (ground), or are enhanced by their contrast with the background. This is true of musical motifs, artwork, dandelions on your lawn, etc. There are implications for your neural processing, apparently making it a good field to study.
Now jazz. Miles and others who play jazz don't read off sheet music when they improvise. They listen and hear the harmonic structure so they can fit into it, then they compose their part on the fly, carefully avoiding the named melody while at the same time trying to express the essence of it. But it's hard to write your own part, always being creative while filling a 3 hour gig with music. Some people do it by using a lot of standard licks, either their own or quoted from great players, occasionally finding their own creative voice in the middle. Others rigorously avoid "licks," Miles was famous for disdaining anyone who used licks. Some just run scales while they wait for inspiration.
It is my theory that nobody can compose high quality original melody fragments continuously in real time. So what most workaday improvisers do is produce a lot of (back)ground, interspersed with a few fragments of figure. Most of the background is scales or modal scales, and the figure usually follows the rules of melody. The greats produce a higher ratio of figure to ground than the rest of us. But if there is not some distinction between figure and ground, the figure gets lost in the noise and the audience tunes out. Miles was not as busy as some players, so he played some figure that was set off by silence instead of low demand running the changes.
What he might have meant by the quote is play more figure and less ground. But I'm not sure. He was not really an intellectually oriented person, he was a performer rather than a thinker.